The toilet paper test is a favorite of mine, especially if I find that a guy has exaggerated his size. If he easily fits in the tube when he is hard, he has a skinny penis. If it doesn't fit, he is extra thick. If it just fits in, he is average.
If it fits loosely, I like to check his length with it too. If the head doesn't poke out of the tube, he has a tiny penis as well as a skinny one. Like I said, I only do this to guys that tell me they are hung and then I find out later they were lying to me. Remember boys, honesty is the best policy in these matters. Most girls don't need you to be a porn star. If you have a lil' one, just spend more time on us before you do the deed. ;>